The purpose of this study is to define mechanisms of abnormal body surface surface potentials and arrhythmias in man. Methods for recording body surface potentials and extracellular cardiac potentials will be used. Cardiac potentials and activation sequence maps will be related to abnormalities in cardiac structure and function. These methods will be used to examine specific problems in two parallel research areas: a) Project 1 - Mechanisms of abnormal QRS axis and duration; and b) Project 2 - Mechanisms of cardiac arrhythmias. Project 1 currently deals with the problem of left axis deviation in man. For this study it is proposed that the mechanisms of abnormal superior QRS are multiple and involve abnormalities in intra-myocardial conduction, developmental variations in myocardial geometry, in addition to abnormalities in Specialized Ventricular Conduction. In Project 2 the studies are based on the assumption that each different cardiac arrhythmia has an unique anatomic-functional substrate, and that the structural or fixed component as well as the interactive, and dynamic component can be defined by cardiac manning and anatomic studies. Currently, Atrial Flutter has been related to an abnormality in atrial conduction resulting from an abnormality in atrial structure. In both research areas, multiple simultaneous cardiac potentials will be recorded. Detailed epicardial activation maps will be made on a beat to beat basis. In addition, multi-electrode needles containing from 15 to 30 points are used to record electrograms at 1 mm intervals from endocardium to epicardium. These data obtained from deeper myocardial structures are essential to defining mechanisms of abnormal QRS potentials or ventricular arrhythmias. These methods will be used to study animal models with spontaneous heart disease and also patients at the time of corrective cardiovascular surgery. Earlier observations in animal and human subjects with congenital heart disease such as the Ostium Primum Atrial Septal Defect, Wolff-Parkinson-White Syndrome, and currently Atrial Flutter indicate the feasibility of this approach for elucidating mechanisms of the abnormal ECG and cardiac arrhythmias.